In some applications of RF power amplifiers, noise can create problems. For example, in wireless communication systems, noise specifications are challenging. Some of the more difficult specifications to meet are for transmit noise appearing in the receive band. The ETSI GSM specification, for example, requires no more than −79 dBm noise power (measured in a 100 kHz bandwidth) between 935 MHz and 960 MHz, and the ETSI DCS specification requires no more than −71 dBm from 1805 MHz to 1880 MHz. Regardless of which specification a power amplifier is designed for, meeting the noise requirements of the specification is difficult.
Various factors make it difficult to meet the stringent noise specifications mentioned above. Power amplifiers having multiple nonlinear stages make the noise specification difficult to meet, since each gain stage contributes to the noise, and each stage can amplify and upconvert the noise of a previous stage. In addition, power amplifier designs that have a lot of active devices in the signal path, especially if slow devices are used, contributes to noise. In some power amplifier designs, it is not always sufficient to minimize the noise generated in the input stages. In these designs, it is also necessary to minimize the noise transfer function from the input stage to the output.